The most simple way is (as you mentioned) to put the DJ Players and the Cartwall Players to a different mixer channel.
Note, that you can create as many mixer channels as you like. Either logical ones (NoSound Device) or physical sound card outputs.
You can then define the routing individually for the DJ Players and the Cartwall Players.
Both the routing and the definition of the mixer channels can be done in the general settings, see section ‘IO’ and ‘Routing’.

For lowering the volume, can then define mixer channel which one to use for the ‘Talkover’ function. Eg. enable it for the DJ Players, but not for the Cartwall.